1. Field of the Invention.
The invention relates to apparatus for injecting gas into molten metal and, more particularly, to a technique for supporting a porous, ceramic, gas-dispersing body such that cracks, with attendant gas leakage, are eliminated.
2. Description of the Prior Art.
In the course of processing molten metals, it sometimes is necessary to treat the metals with gas. For example, it is customary to inject gases such as nitrogen, chlorine, and argon into molten aluminum and molten aluminum alloys in order to remove undesirable constituents such as hydrogen gas, non-metallic inclusions, and alkali metals. The gases added to the molten metal chemically react with the undesired constituents to convert them to a form (such as a precipitate, a dross, or an insoluble gas compound) that can be separated readily from the remainder of the molten metal.
As used herein, reference to "molten metal" will be understood to mean any metal such as aluminum, magnesium, copper, iron, and alloys thereof, which are amenable to gas purification. Further, the term "gas" will be understood to mean any gas or a combination of gases, including argon, nitrogen, chlorine, freon, sulfur hexafluoride, and the like, that have a purifying effect upon molten metals with which they are mixed. The process of introducing purifying gas into molten metal is referred to variously as "gas injection" or "degassing."
In order to efficiently carry out a gas injection operation, it is desirable that the gas be introduced into the molten metal in the form of a large number of extremely small bubbles. As the size of the gas bubbles decreases, the number of bubbles per unit volume increases, and thus the total surface area per unit volume increases. An increase in the number of bubbles and their surface area per unit volume increases the probability of the gas being utilized effectively to purify the molten metal.
One known technique for introducing gas into molten metal consists of lining a portion of a molten metal-containing vessel (preferably the bottom of the vessel) with a porous ceramic body. The gas is introduced into the body at a location remote from the metal-contacting surface of the body. During its passage through the body, the gas follows a number of small, tortuous paths such that a large number of small bubbles will be issued into the molten metal. Porous ceramic bodies have been used as described for the purification of molten metal, and are commercially available from North American Refractories Company (NARCO), Cleveland, Ohio 44115, under the trademarks A-94 and MAS-100.
In the referenced NARCO apparatus, the porous ceramic body is supported by a metal casing that acts as a manifold to introduce gas into the body. Typically, the casing is made of mild steel (for use with argon or nitrogen) or inconel (for use with chlorine or freon). The assembled body/casing is surrounded and supported by a nest brick comprised of a refractory material such as low-cement alumina castable. The nest brick includes an opening through which a surface of the body is exposed for the discharge of bubbles into molten metal. The assembled body, casing, and nest brick are supported within a molten metal container such as a furnace, ladle liner, ladle, or filter box, usually by being cast in place by means of a refractory material such as low-cement alumina castable.
A problem with the foregoing construction is that it is difficult to maintain an effective gas seal between the casing and the body, and between the casing and the nest brick. The difficulty arises in part because the coefficients of thermal expansion of the metal casing and the refractory materials are considerably different; also, the metal casing is subject to attack if chlorine is the gas being used. If a crack should develop (as used herein, the term "crack" refers to any defect in the gasdispersing apparatus that permits undesired gas leakage), gas will leak through the crack, and thereafter frequently will migrate through the nest brick and refractory support to the ambient atmosphere. Gas migration through 20 inches or more of refractory material is possible. The problem is particularly acute in the case of chlorine due to the harmful effects of chlorine upon release into the atmosphere. The problem also is undesirable if argon is being used due to the relatively great expense of argon. Regardless of the type of purifying gas being used, it is important that cracks be prevented so that gas leakage will be prevented.
Desirably, a technique would be available for injecting gas into molten metal that would accomplish the objectives of dispersing a large number of exceedingly small bubbles into the molten metal while, at the same time, avoiding cracks in the gas dispersing apparatus that result in gas leakage. It also would be desirable for any such apparatus to be capable of being manufactured easily, at reasonable expense. Further, it would be desirable that any such gas injection apparatus be usable with existing equipment such as ladles, furnaces, and the like, with no modification, or with only minor modification, of the existing equipment.